IS 3400 (Par! 20) : 1994 IS0 1431-l : 1989 ( Reaffirmed 2003 ) Indian Standard METHODS OF TEST FOR VULCANIZED TO OZONE CRACKING - RUBBERS PART 20 RESISTANCE STATIC STRAIN TEST ( First Revision ) UDC 678.43:620.193.3(661.94) 0 BIS 1994 BUREAU MANAK OF BHAVAN, INDIAN STANDARDS ZAFAR MARG 9 BAHADUR SHAH NEW DELHI 110002 April 1994 Price Group 4 Rubber Products Sectional Committee, PCD 13 EXPLANATORY NOTE This Indian Standard (First Revision) was adopted by the Bureau of Indian Standards after the draft finalized by the Rubber Products Sectional Commitee, approved by the Petroleum, Coal and Related Products Division Council. This standard was first published in 1977 based on ISO/R-1431:1972 `Vulcanized rubbers - Determination of resistance to ozone cracking under static conditions' and ASTM D 1149 - 64 `Accelerated ozone cracking of vulcanized rubbers'. The method prescribed in this standard may not give results correlating exactly with outdoor exposure tests since correlation of accelerated ozone tests with performance is in general not good and is highly dependent upon the specific conditions of both the accelerated and outdoor exposures. Conditions that influence accelerated test are ozone concentration, relaxation of stress, temperature and degree of bloom of additives. Conditions that influence outdoor tests in addition to these are the amount of sunshine and rainfall. The present revision has been taken up as a result of review of the standard in the light of experience gained over the years. The Committee had decided to revise this standard under dual numbering. NATIONAL FOREWORD This Indian standard IS 3400 (Part 20) : 1994 which is an adopted version of IS0 1431-1 : 1989 :`Rubber vulcanized orthermoplasticResistance to ozone cracking-Pan 1 : Staticstrain test' issued bythe International Organization for Standardization (ISO), has been prepared on the recommendation of the Rubber Products Sectional Committee, PCD 13. In the adopted standard, certain terminology andconventions are not identical with those used in Indian Standards. Attention is specially drawn to the following: a) b) Comma (,) has been used as decimal mark while in Indian Standards the current practice is to use a point (.) as the decimal marker. Wherever the words `International Standa:d' appear referring to this standard, they should be read as `Indian Standard'. are referred to read in their respective place the following: Corresponding Indian Standard Degree of Correspondence In this standard, the following Internationafstandards International Standard IS0 471 : 1983 Rubber -- Standard temperatures, humidities and time for the conditioning and testing of test pieces IS0 1431-2 : 1982 Rubber vulcanized - Resistance to ozone cracking-Part 2 Dynamic starain test IS0 4661-l : 1986 Rubber, vulcanized - Preparation of samples and test pieces - Part 4 Physical tests IS 13867 : 1993 Rubber - Standard temperatures, humidities and times for conditioning and time interval between vulcanization and testing of test pieces - Techinically equivalent _- IS 3400 Methods of test for vulcanized rubbers: (Pan 1) 1987 Tensile stress-strain properties (second revision) (Part 2) 1980 Hardness (first revision) (Part 4) 1987 Accelareted ageing (first revision) Techinically equivalent (Pan 9) 1978 Density (first revision) (Part 10) 1977 Compression set at constant strain (first revison) (Continued on third cover) IS 3400 ( Part 20 ) : 1994 IS0 1431-l : 1989 Indian Standard METHODS OF TEST FOR VULCANIZED RUBBERS PART 20 RESISTANCE TO OZONE CRACKING STATIC STRAIN TEST ( First Revision ) 1 Scope IS0 4661-l : 1%6, Rubber, - vulcanized - Preparation of samples and test pieces Part 1: Physical tests. This part of IS0 1431 specifies a method for the determination of resistance of vulcanized or thermoplastic rubbers to cracking when exposed, under static tensile strain, to air containing a definite concentration of ozone and at a definite temperature in circumstances that exclude the effects of direct light. Great caution is necessary in attempting to relate standard test results to service performance since the relative ozone resistance of different rubbers can vary markedly according to conditions, especially ozone concentration and temperature. In addition, tests are carried out on thin test pieces deformed in tension and the significance of attack for articles in service may be quite different owing to the effects of size and the type and magnitude of deformation. Explanatory notes on the nature of ozone cracking are given in annex A. Methods for determining resistance to ozone cracking under dynamic strain conditions and combined dynamic. and static strain conditions are specified in IS0 1431-2. A reference method for estimating the ozone concentration will form the subject of IS0 1431-3. 3 Definitions For the purpose of this part of IS0 1431, the following definitions apply. 3.1 threshold strain: The highest tensile strain at which a. rubber can be exposed at a given temperature to air containing a given concentration of ozone without ozone cracks developing on it after a given exposure period. Threshold strain must be distinguished from limiting threshold strain, defined in 3.2, 3.2 limiting threshold strain: The tensile strain below which the time required for the development of ozone cracks increases very markedly and can become virtually infinite. 4 Principle 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this part of IS0 1431. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this part of IS0 1431 are encouraged to investigate the possibility of applying the most recent editions of the standards listed below. Members of IEC and IS0 maintain registers of currently valid International Standards. IS0 471 : 1983, Rubber Standard temperatures, humidities Test pieces are exposed under static tensile strain, in a closed chamber at a constant temperature, to an atmosphere containing a fixed. concentration of ozone. The test pieces are examined periodically for cracking. Three alternative evaluation procedures are described for selected values of ozone concentration and exposure temperature : A Determination of the presence or absence of cracks, and if required, an estimate of the degree of cracking, after exposure for a fixed period of time at a given strain. ' B Determination of time to the first appearance of cracks at any given strain. C Determination of the threshold strain for any given exposure period. and times for the conditioning and testing of test pieces. Resistance to ozone IS0 1431-2: 1982, Rubber, cracking Part 2: Dynamic vulcanized strain test. IS 3400 ( Part 20 ) : 1994 IS0 1431-1 : 1989 5 Apparatus (see figure 1) The use of oxygen is necessary when using the discharge tube in order to avoid the formation of nitrogen oxides. The ozonited oxygen or air may be diluted with air to attain the required ozone concentration. Air used for generation of ozone or dilution shall first be purified by passing it over activated charcoal and shall be free from ozone concentration, The temperature within f 2 "C. of any contaminants likely to affect of ozone. kept constant to the cracking or estimation the source shall be WARNING - Attention is drawn to the highly toxic nature of ozone. Efforts should be made to minimize the exposure of workers at all times. In the absence of more stringent or contrary national safety regulations in member body countries, it is recommended that 10 parts of ozone per hundred million parts of air of the surrounding atmosphere by volume be regarded as an absolute maximum concentration whilst the maximum average concentration should be appreciably lower. NOTE An exhaust vent to remove ozone-laden air is advised. The ozonized air shall be fed from the source into the chamber via a heat exchanger to adjust its temperature to that required for the test humidity and shall be brought to the specified relative (see 8.3). 5.1 This Test chamber. shall be a closed, to non-illuminated within f chamber, thertest 5.3 Means for adjusting the concentration of ozone This may be, but does not have to be, automatic. When an ultra-violet light source is used the amount of ozone mostatically controlled 2 OC of the temperature, lined with, or constructed of, a material (for example, aluminium) that does not readily decompose ozone. Dimensions shall be such that the requirements of 5.5 are met. The chamber may be provided with a window through which the surface of the test pieces can be observed. A light to examine test, pieces may be installed. produced can be controlled by adjusting the voltage applied to the tube or the gas flow rates, or by shielding part of the tube exposed to the gas flow. When a silent discharge tube is used, the amount of ozone produced can be controlled by adjusting the voltage applied to the generator, the dimensions of the electrodes, two-stage the oxygen flow rate, or the diluent air flow rate. A dilution of the ozonized air can also be used. The adshall be such that they will maintain that the test chamber within is opened 30 min. The the concenafter each or inof 5.2 Source of ozonized air apparatus may be used: justments occasion tration within the tolerances given in 8.1. In addition, Either of the following spection of test pieces, the ozone concentration a) b) an ultra-violet lamp; tube. the test concentration ozone entering a silent discharge centration specified the chamber for insertion shall return to the con- concentration shall at no time exceed for the test. FLOWMETER I REGULATOR l-l PURIFYING COLUMN , HOLE FOR TEST SAMPLE r- FLOWMETER --c ,17 t MANOMETER _-----__- -WASHING FLASKS L TEMPERATURE INDICATOR OZONIZER HEAT EXCHANGER I I-----l Figure 1 Schematic diagram of the apparatus 2 IS 3400 ( Part 20 ) : 1994 IS0 1431-1 : 1989 5.4 Means of determining the ozone concentration A means of sampling the ozonized air from the vicinity of the test pieces in the chamber and means of estimating its ozone content shall be provided. The methods for estimating the ozone concentration will form the subject of IS0 1431-3. 5.5 Means of adjusting gas flow A mechanism shall be provided which is capable of adjusting the average velocity of flow of ozonized air in the test chamber to a value of not less than 8 mm/s and preferably to a value between 12 mm/s and 16 mm/s, calculated from the measured gas flow rate in the chamber divided by the effective crosssectional area of the chamber normal to the gas flow. In tests intended to be comparable, the velocity shall not vary by more than f 10 %. The gas flow rate is the volume throughput of ozonized air in unit time and this shall be sufficiently high to prevent the ozone concentration in the chamber being significantly reduced owing to ozone destruction by the test pieces. The rates of destruction will vary according to the rubber being used, the test conditions and other details of the test. As a general guide, it is recommended that the ratio of the exposed surface area of the test pieces to the gas flow rate should not exceed 12 s/m, but this may not always be low enough. In cases where there is doubt, the effects of destruction should be checked experimentally and, if necessary, the test piece area should be decreased. A diffusing screen or equivalent device should be used to assist thorough mixing of incoming gas with that in the chamber. If high velocities are desired, a fan may be installed in the chamber to raise the velocity of flow of ozonized air to 600 mm/s + 100 mm/s. NOTE - The ratio, expressed in seconds per metre, is derived frbm surface area in square metres and vnllm&tric flow rate in cubic metres per second. Figure 2 - Path of test piece and swept area (shaded) 6 6.1 Test piece General Standard test pieces shall be strips or dumb-bells as specified in 6.2 and 6.3. Test pieces shall be cut from freshly moulded sheet or, if required, from a finished product in accordance with IS0 4661-1. Test pieces shall have an undamaged test surface; ozone l&stance shall not be assessed on surfaces that have been cut or buffed. Comparisons of different materials are only valid if the cracking is assessed on surfaces of similar finish produced by the same method. For each test condition, at least th!ree test pieces shall be used. 6.2 Strip test piece 5.6 Test piece carrier for holding the test pieces at the re- Clamps shall be provided quired elongation with both sides in contact with the ozonized air in such a manner that the length of the test piece is substan- tially parallel to the gas flow. The clamps shall be made of a material which does not readily decompose ozone (for example aluminium). The use of a mechanically rotating carrier mounted in the test chamber and upon which the clamps or frames for holding the test pieces are mounted is recommended to equalize the effect of different ozone concentrations in the chamber. In one example of a suitable carrier, the test pieces move at a speed between 20 mm/s to 25 mm/s in a plane normal to the gas flow and each follow consecutively the same path in such a manner that the same position within the chamber is visited by the same piece every 8 min to 12 min, and the area swept by the piece (shown shaded in figure 2) is at least 40 % of the available cross-sectional area of the chamber. The test piece shall consist of a strip of not less than 10 mm width, thickness 2,0 mm f 0,2 mm, and length not less than 40 mm between the grips before stretching. The ends of the test piece held in the grips may be protected with an ozone-resistant lacquer. Care shall be taken in selecting a lacquer to ensure that the solvent used does not appreciably swell the rubber. Silicone grease shall not be used. Alternatively, the test piece may be provided with modified ends, for example by the use of lugs, to enable it to be extended without causing excessive stress concentration and breakage at the grips during ozone exposure. IS 3400( Part 20): ISO 1431-1:1989 1994 6.3 Dumb-bell test piece Sampies and test pieces shall be stored in the dark, in an essentially ozone-free atmosphere during the normal the period between but Other, vulcanization and stretching; storage temperature The test piece shall consist of a strip of 5 mm width and 50 mm length, between enlarged tab ends 12 mm square (see figure 3). This test piece sha{i not be used for procedure A. 5 should be the standard temperature (see ISO 471], Dimensions in millknetres 12 for particular applications. These storage conditions should be used, as far as possible, for products. For evaluations intended to be comparable, the storage time and conditioris shall be identicat. For thermoplastic rubbers, the storage period shall begin im- controlled, tamparaturas may be used if appropriate [ 1 I I mediately after shaping. 7.2 Conditioning in the strained state After stretching, the test pieces shall be conditioned for a period of between 48 h and 96 h in an essentially ozone-free atF@re NOTES 3 ­ Dumb-bell test piece mosphere in the dark; the temperature for this conditioning (see ISO 471), but shalt normally be the standard highly temperature 1 It is recommendedthat test sheets are moulded between polished eluminium other temperatures may be used if appropriate for particular applications. The test pieces shail not be touched or otherwise disturbed in any way during the conditioning period. For tests intended to be comparable, the conditioning time and ternparatwe shai! be the same. foil which is left on the rubber until the test pieces are prepared. This provides protection against handling end ensures a fresh test sutiace at the time of testing. 2 It is sometimes impracticable to cut the standard test pieces. In such cases one form of test piece which maybe used is the T 50 dumb. bell with a length of !iO mm snd width 2 mm. When used to detect the onset of cracking these test pieces have bean shown to give approximately equivalent results to the standard test pieces at the aarne percentage 3 e!orrgations. to the exposure of test pieces at several different 8 8.1 Test conditions Ozone concentration ozone con- As an alternative strains, a test piece in the form of an zrnnulus has been used which is strained to produce a continuous been found to give approximately test pieces when used to detenmine range of extensions. equivalent threshold s?rsin. This method has results to the standard The test shall be carried out at one of the following centrations, expressed in parts of ozone per hundred million of air by volume (pphm): 25 pphm 50 pphm 100 pphm 200 pphm * 3 t 5 pphm 5 pphm 10 pphm 4 As another alternative to the exposure of test pieces at several different strains, a rectangular test piece in the fcrm of a bent loop can be used to provide a gradation of extensions within one test piece. k 20 pphm specified, for testing 7 7.1 Conditioning Conditioning in the unstrained state Unless otherwise tration is required the test shal[ be carried ~ 5 pphm. If a lower rubbers out at an ozone concentration low ambient of 50 pphm concen- ozone concentrations, known to be used under an ozone concentration of For all test purposes, the minimum time between vulcanization 25 pphm + 5 pphm is recommended. are bekrg tested, a test concentration 200 pphm NOTE ­ If highly resistant polymers of 100 ppkrm i- 10 pphm or and straining the test pieces shaH be 16 h. For non-product tests, the maximum time between viilcanization and straining the test pieces shall be 4 weeks. For product tests, wherever possible, the time between vulcanization and straining the test pieces shall not be more than 3 months. In other cases, teats shall be made within 2 months of the date of receipt of the product by the customer, T 20 pphm is recommended. in atmospheric pressure It has berm found that differences expressed of the can influence ozone cracldng when test pieces are e xposett to constanl ozone concentrations ozorized pressures. temperature cir At in parts per hunrfred million. This efthe ozone content of ozone, in !he i.e. in partial pressure fect may be taken into account by expressing in terms standard rr%llipascals, and makmg comparisons conditions of at constant atmospheric ozone partial pressure and wXI be given m {101 kPa, 273 K i, a concentration of 1 pphm is equivalent Test pieces and test sheets shall not, between vulcanization into contact w-d insertion with rubbers in the cabinet, of a different the time of This is be allowed to come composition. to a partial pressure of 1,01 mPa. Further guidance 1s0 1431-3. necessary to prevent additives which may affect the development G. ozone cracks, such as antiozonants, from migrating by diffusion from one rubber into adjacent rubbers, R is recommended pieces and sheets that a}uminium of different migration foil be placed between test 8.2 Temperature of test shall be 40 `C k 2 `C or 23 `C ~ 2 `C. Other ? 2 "C may be service The preferred temperature temperatures such as 30 `C used if they are more representative environment, but the results obtahed tained at 40 `C ~ 2 `C, of the anticipated compositions, but any other will differ from those ob- method which prevents of additives can be used. 4 IS 3400 ( Part 20) : 1994 IS0 1431-1 : 1989 NOTE - For applications where markedly varying temperatures may be encountered it is recommended that two or more temperatures, covering the service range, be used. elongation and an alternative exposure period may be given in the appropriate material specification. 9.3 8.3 Relative humidity Procedure B given Strain the test pieces at one or more of the elongations in 8.4 and condition The relative humidity of the ozonized air should not normally be more than 65 % at the test temperature. elongation specified. thereafter appearance is used, them in accordance this shall be 20 % unless with 7.2. If only one otherwise intervals Examine the test pieces after 2 h, 4 h, 8 h, 16 h, 24 h, if necessary, at suitable in the test chamber and note the time until the first 48 h, 72 h and 96 h and, Very high humidity can influence the results; when applicable, for products intended for use in damp climates,,the test shall be carried out at a relative humidity in the range 80 % to 90 %, if this is practicable. of cracks at each elongation. 9.4 Procedure C 8.4 Elongation be carried out with test pieces stretched percentage elongations: Strain the test pieces at no fewer than four of the elongations given in 8.4 and condition them in accordance with 7.2. Examine the test pieces after 2 h, 4 h, 8 h, 16 h, 24 h, 48 h, 72 h and 96 h and, if necessary, test chamber estimated. NOTE For procedures B and C it is sometimes satisfactory to omit Tests should normally at suitable intervals thereafter so that the threshold in the of to one or more of the following and note the time until the first appearance cracks at each elongation 5+1 20 + 2 50+_2 10 * 3Of2 60+2 1 15 * 40+2 80+2 2 strain can be examination after 16 h. NOTE - The elongation(s) used for procedures in service. A and B should be similar to those anticipated 10 10.1 Expression Procedure of results A or cracking. If cracking has 9 9.1 Test procedure Report the results as no cracking occurred and an estimate of the degree of cracking is required, a description of the cracks (for example, appearance of single General the rate of flow cracks, the number of cracks per unit area and the average length of the 10 largest cracks) may he given, or a photograph Adjust and temperature to that of the ozonized gas of the cracked test piece may be taken. and its ozone concentration required and place the strained test pieces, suitably conditioned, Maintain the test conditions in the test chamber. 10.2 Procedure B of cracks as the measure strain. at the required levels. Take the time to the first appearance of ozone resistance at the specified Periodically examine the test pieces for the development of cracking by means of a lens of magnification about X 7, the test pieces being illuminated dow in the chamber from the chamber examination. at the fime of examination by a suitably arranged light source. The lens may either be mounted in a winwall, or the test pieces may be removed in their clamps. The test or bumped when carrying out the for a short period, 10.3 Procedure C Indicate the range within which the threshold strain is found to lie by reporting the highest strain at which cracking was not detected and the lowest strain at which cracking was observed exposure period. If replicate tests give diffor example, range observed, after the specified pieces shall not be handled ferent results, quote the extreme NOTE Cracking on surfaces which have been cut or buffed should be ignored. if three test pieces are used at each of 10 %, 15 % and 20 % strains and one cracks at 10 % only, one at 15 % and all three at 20 %, the quoted range should be 10 % to 20 %. Graphical presentation may be used to.assist interpretation of the results. Three alternative procedures for exposure of test pieces are NOTES 1 A method that has been found useful is to plot the logarithm of strain against the logarithm of the time to first cracking - both the longest time at which no cracks are seen and the earliest time when permissible. 9.2 Procedure A condition them in acafter 72 h in the test An alternative Strain the test pieces at 20 % elongation, cordance chamber with for cracks are observed may be plotted. Where possible, a smooth curve may be drawn taking into account the gap between the longest time with no cracks and the earliest time with cracks at each strain to assist 7.2, the and examine development them of cracking. estimation of the threshold strain for any time within the test period 5 IS 3400 ( Part 20) : 1994 IS0 1431-l : 1989 (see figure 4). For some rubbers the curve may approximate to a straight line but this should not be assumed since it can lead to large errors in estimating threshold strain. Unless otherwise specified, the threshold strain at the longest test period should be reported. 2 With some rubbers, a linear plot of strain against time to first cracking will enable the existence of a limiting threshold strain to be observed. c) test details : 1) the ozone concentration and the method of estimation; 2) the temperature of the test; 3) the temperature of conditioning, if other than the standard temperature; 4) the humidity, if other than specified; the air flow rate; the strain(s) on the test pieces; the duration of the test; any non-standard procedures; 11 Test report 5) 6) The test report shall contain the following information: a) sample details : 1) 2) a full description of the sample and its origin; compound identification; d) 7) 8) test results : 1) the number of the test pieces tested at each strain; 2) for procedure A only, whether cracking occurred. If required, the nature of cracking may also be given; 31 for procedure 6 only, the times to the first appearance of cracks; 4) for procedure C only, the observed range of threshold strain for a suitable exposure period or periods, or the limiting threshold strain; e) the date of the test. 3) method of preparation of test pieces, for example, whether moulded or cut; b) test method : 11 a reference to this part of IS0 1431; 2) 3) 4) the procedure used (A, 6 or C); the type and dimensions of the test piece; whether a rotating carrier was used; LAST WITH FOR THE EXAMPLE SHOWN THRESHOLD STRAIN 4Uh = 10% NO CRACKING 1 2 4 Figure 4 - 8 24 48 96 TIME, h (LOG SCALE) Results in graphic form 6 IS 34OO(Part20):1994 IS0 1431-1:1989 Annex A (informative) Explanatory notes Cracks develop according in rubber only on surfaces subjected and nature to tensile vary the whole relationship is determined. between strain and severity of cracking strain. The pattern of cracks, and the severity of cracking, to the magnitude of the applied strain. The way in which ozone cracking depends on strain is not a simple relationship. The number of cracks on a test piece is related to their size and this relationship depends on the threshold strain for a given exposure period and the elongation The strainon an article in service will vary from a minimum at one point, which need not necessarily be zero, to a maximum at some other point. The pattern this range should be considered measured. The first criterion for describing total freedom threshold from cracking. strain for a given exposure a test piece resistance. However, at a given a material as ozone-resistant Thus, period is of cracks at all extensions in when ozone resistance is being applied to the test piece, for any given material. Thus no ozone cracking will occur for a given exposure at strains between zero and the threshold (by definition). cracks, above which the will be large, and will be found the cracks threshold, will become period A few more the higher the threshold or the higher the limiting the better is the ozone at strains slightly strain or the longer the time before cracks appear on elongation, numerous and smaller at progressively higher strains. At very high strains the cracks may sometimes be so small as to be invisible to the naked eye. an alternative criterion may be necessary when on the Cracks will coalesce as the exposure increases, particularly ozone cracks below a certain limit of size are permitted the concept that one rubber can be described when they are very numerous but without a proportionate on the surface of the test piece. increase in depth. Coalescence is rubber over a given range of strains. This criterion is based on as more ozone resistant than another if the ozone cracks on it are less severe This will result in the length of some cracks being increased, probably due to a tearing process as well as ozone attack, and will sometimes result in a number of larger cracks being scattered among the general mass of small dense cracks which often cover the test piece surface at high strains. over the range of extensions encountered in service, which should be specified. The visual nature of the ozone cracks which develop in the test piece should then be reported so that (Continued from second cover) For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed orcalculated, expressing the result of atest oranalysis, shall be rounded off in accordance with IS 2:1960 `Rules for rounding off numerical values (revised)`. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard. Bureau of Indian Standards BIS is a statutory institution established under the Bureau of Indian Standards Act, 1986 to promote harmonious development of the activities of standardization, marking and quality certification of goods and attending to connected matters in the country. Copyright BIS has the copyright of all its publications. No part of these publications may be reproduced in any form without the prior permission in writing of BIS. This does not preclude the free use, in the course of implementing the standard, of necessary details, such as symbols and sizes, type or grade designations. Enquiries relating to copyright be addressed to the Director (Publication), BIS Revision of Indian Standards Amendments are issued to standards as the need arises on the basis of comments. Standards are also reviewed periodically; a standard along with amendments is reaffirmed when such review indicates that no changes are needed; if the review indicates that changes are needed, it is taken up for revision. Users of Indian Standards should ascertain that they are in possession of the latest amendments or edition by referring to the latest issue of `BIS Handbook' and `Standards Monthly Additions'. This Indian standard has been dcvclopcd from DCX No. PCD 13 (1314)\ ---____Amend ho. 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